Known in the art are different clutch types that are aimed at protecting a torque transmission against a sudden overload occurring between a driven member and a drive member. When such an overload occurs, there is a need to disengage the drive member in order to avoid any permanent damage to the drive mechanism. The overload clutches known from prior art have low or medium torque capacities. One will find three main basic principles behind the design of the clutches from prior art.
The principle behind the first type of overload clutches is the use of a shear pin that allows a drive line subjected to an overload to disengage. An example of such an overload clutch is given in U.S. Pat. No. 6,398,654. This patent discloses an overload clutch with a shear pin element, interconnecting a drive shaft and a driven chain sprocket. The overload clutch includes a first clutch element fixed for rotation with the shaft, a second clutch element mounted on the shaft next to the first clutch element, for axial and rotation of movement relative to the first clutch element. The second clutch includes a circular flange disposed against a flat circular surface of the sprocket and coupled for rotation therewith by a shear pin extending through aligned holes in the flange and sprocket. The first and second clutch elements respectively have positive engaging surface elements that are normally held in locking engagement which each other by a coil compression spring acting between the shaft and this sprocket. When the clutch is subjected to an overload, the shear pin shears.
The principle behind the second type of overload clutches resides in the use of slippage between friction discs that cooperate with each other to engage and disengage the clutch. When a drive train is subjected to a torque overload, the discs are caused to slip one with respect to each other, thereby causing the clutch to disengage. An example of a slip clutch is given in U.S. Pat. No. 5,807,179 which discloses a torque overload coupler for a driveline that allows limited free motion. In one embodiment, the torque overload protection is provided by a friction slip clutch and tangs form at the inner parameter of a clutch plate fit into spaces between logs defined on a hub to provide a limited range of free motion. In one aspect of the disclosed invention, a first means for connecting the copier to a torque input shaft and a second means for connecting the coupler to a torque output shaft have a bearing between them for allowing relative rotation and fixing them axially. A clutch housing is fixed to the first connecting means and a clutch plate in the clutch housing which engages the housing as tangs defined at an inner parameter with interstitial angular spaces between the tangs. A pressure plate is provided in the clutch housing which is biased against the clutch plate so that the clutch is normally engaged. Under operation, when the device is being used to transmit torque in the operation of the driveline, the torque is transmitted by the logs abutting the tangs and is transmitted up to the torque value at which slippage occurs between the clutch plate and the housing and pressure plate. One drawback with slip clutches is that, as time goes by, they wear out and they either have to be replaced or remachined.
The third type of clutches known in prior art, which makes use of spherical couplings housed between two circular coaxial clutch elements, has a very low capacity. An example of such a clutch is givent in U.S. Pat. No. 4,467,902. This patent discloses an overload clutch where two co-axial clutch members, one driving and one driven, are normally coupled for joint rotation by several peripherally spaced rotary coupling elements projecting under a resilient biasing force into aligned sets of recesses thereof, at least one of these sets having camming edges by which the coupling elements are dislodged against their biasing force when the driven clutch members tend to lag behind the driving clutch member in the event of an overload. U.S. Pat. No. 5,672,110 gives another example of an overload clutch of that type.
Also known in the prior art, there is U.S. Pat. No. 6,471,024 B2 which discloses a torque limiting mechanism for transmitting power from an external drive source to the drive shaft of a compressor. A pulley which is coupled to the external drive source, has elastic members. A hub, which is attached to the drive shaft, has engaging portions. A coupler member is located between the pulley and the hub. The coupler member is engaged with the elastic members and with the engaging portion such that power is transmitted from the pulley to the hub. The urging members urge the coupler member such that the coupled member is disengaged from the engaging portion. Once power is transmitted from the pulley to the hub, the elastic members maintain the coupler engaged with the elastic members and the engaging portion. When load generated due to power transmission exceeds a predetermined level, the elastic members are deformed such that the coupler member is disengaged from the elastic members. When disengaged from the elastic members, the coupler member is disengaged from the engaging portions.
A further type of overload clutches is disclosed in U.S. Pat. No. 4,846,894 which describes a pneumatic clutch apparatus. The disclosed apparatus is aimed to be disposed at the rear of a transmission and has a pneumatic actuator for engaging and disengaging the clutch. The clutch comprises a flywheel connected to an output shaft of the engine, a clutch disk connected to an output shaft of the clutch, a pressure plate and a clutch cover. The clutch cover is fixed at the outer periphery of the outer peripheral portion of the flywheel. A pneumatic actuator is fixed to the clutch cover. The actuator is designed to force the pressure plate, so that the plate may press a friction facing at the radially outer portion of the disk against the flywheel to engage the clutch. The actuator is located between the pressure plate and the clutch cover. The actuator is provided with a cylinder fixed to the cover, at this end fitted into the cylinder and a case surrounding the cylinder. The piston is fixed to bosses of the plate with a heat insulator and the case in between. A pressure chamber is formed between the piston and the cylinder. When a pneumatic high pressure is supplied into the chamber, the piston forces the plate toward the flywheel. The pressure chamber is connected through an air passage mechanism to a pneumatic pressure control mechanism disposed at the outside of the housing. The pneumatic pressure control mechanism is so constructed that the compressor is connected through an air tank and a pressure regulator valve to a pressure control valve. The compressor is adapted to be driven by the engine. The regulator valve regulates the pneumatic pressure to be supplied to the control valve to a constant value. When the full pressure is introduced into the chamber, the clutch is engaged.
When the pressure in the chamber is released, the clutch is disengaged. When a low pressure is introduced into the chamber, the clutch takes a half-engaged condition in which the facing is pressed against the flywheel with slipping thereon.
The following documents also disclose different overload clutches: U.S. Pat. Nos. 4,081,063; 4,344,306; 4,004,667; 4,220,230; 4,548,305; 4,746,320; 4,770,281; 4,798,559; 4,875,528; 4,901,610; 5,531,307; 6,059,085; 6,447,397B1; 5,415,575 and 5,030,311.
One drawback with the above-mentioned prior art clutches is that reengagement of the decoupled drive and driven members of most of these clutches requires a deactivation of the driving member and a realignment of their parts. For those where a reengagement can be carried out automatically, they are rather noisy decoupled, owing to a ratchet effect when the spherical coupling bodies pass by their respective recesses as an example. Furthermore, most of the clutches from prior art have a predetermined capacity which can be changed only by dismantling the mechanism and stopping the machinery.
There is therefore still a need for an improved overload clutch which is simple to use for an operator and which also allows an automatic reengagement of the decoupled members after the occurrence of an overload.